Using Barometric Pressure to Determine the Weather

When you listen to the weather reports on TV, you may get a little confused listening to all the jargon; with a little decoding, however, you can start to understand the mechanics of weather. High and low air pressure are the engines that drive good days and foul weather.

Difficulty: Moderate

Instructions

1. When the sun heats the air, molecules heat up and rise, creating a center of low air pressure because there is less mass (meaning fewer molecules) in one particular area. Denser areas are called high air pressure areas because they actually weigh more than low air pressure areas. In high pressure centers, because the air sinks, the air molecules can hold more water; thus they generate nicer days.

2. Low pressure generally brings rain. Low pressure areas tend to cause precipitation because air tends to rise, bringing along moisture which then cools, causing rain or snow. Rain is caused when warm air rises over cool air, releasing water from the air as the warm air rises and subsequently condenses. Also, when a cool front comes in, thunderstorms may happen when cool air causes the moisture in the warmer air to precipitate. Additionally, wind tends to move counter clockwise around pressure centers, and tends to move faster around low pressure centers.

3. Understand wind. Winds are caused when molecules of air try to even out the differences between areas of high pressure and areas of low pressure.

4. Know there are various types of highs and lows. High pressure areas generally cause air centers to fall, creating sunny days with little or no precipitation. However, high pressure areas can occur in cooler areas in the Northern climes, and may be referred to as 'cold anticyclones,' because cool air falls and spirals at the same time; the cold ground drives the air to sink faster. Thermal lows may happen along the ocean as the air is heated, thus creating a local low air pressure system, which can cause storms. This generally happens in the afternoons, as the air heats up over the land, and then precipitates as it rises over the ocean.

5. Understand holding weather patterns. Areas of similar pressure tend to band together and these are called isobars. Stagnant air or the same weather for days in a row may be caused by 'blocking highs' and 'ridges' may be areas of high pressure that are held in place by surrounding areas of low pressure.

Resources

• How barometric pressure works